1. Field of Invention
The present invention relates to signal transmission structure. More particularly, the present invention relates to the transmission structure of high frequency signals.
2. Description of Related Art
Recently, the DSP's internal clock rate has reached GHz level and signal transmitting/receiving frequency is also up to more than 100 MHz. In such high-speed digital circuits, noise and electromagnetic interference (EMI) are critical problems. Unfortunately, digital signal processing (DSP) systems processing audio/video signals and communication signals are sensitive to these disturbances. For example, the high-speed switching signals, with a lot of noise and interference, affect the DSP system performance.
There are several sources which might cause the switching noise. The transmission line with reflection phenomena is one of them. To minimize the reflection phenomena caused by the high frequency transmitting, the length of the current return path is short as possible. Generally speaking, low-speed signals return to the signal source along the shortest path with minimum resistance, and high-speed signals return to the signal source along with the shortest path with minimum inductance. Therefore, the object of high-speed circuit is to provide the minimum inductance path. This can be achieved with the power supply plane and the ground plane. Power plane is an inherent high-frequency decoupling capacitor and is able to minimize parasitic inductance. The ground plane also known as the mirror plane has a shielding effect and provides the shortest current return path.
The existing digital circuit usually requires several power signals to provide different voltages. Therefore, the power plane is usually divided into several regions with slots to provide different voltages on different regions. Because current of the system needs to return to the current source, if there is discontinuation in the region of the system, such as the slot, current needs to go around to pass the discontinuous region, which increases the length of the current return path and the equivalent inductance. Thus, high-frequency signals might be filtered out due to the increased inductance, which causes signal distortion.
Hence there is a need for a new signal transmission structure that can reduce the high frequency signal loss caused by the slot on the power plane and keep the signal complete.